Add MAX_MESSAGE_LIMIT to readme. (#71)

README.md

@@ -20,6 +20,7 @@ export PRIVATE_KEY=0xac0974bec39a17e36ba4a6b4d238ff944bacb478cbed5efcae784d7bf4f
 export ETH_FROM=0xf39fd6e51aad88f6f4ce6ab8827279cfffb92266
 export RLN_RELAY_EPOCH_SEC=10
 export RLN_RELAY_MSG_LIMIT=2
+export MAX_MESSAGE_LIMIT=100  # Contract's message limit. Needs to be higher or equal than RLN_RELAY_MSG_LIMIT, otherwise nodes won't start correctly.
 
 docker-compose --compatibility up -d
 ```